Investigation of Intermittent Gas Lift by Using Mechanistic Modeling

Abstract

ABSTRACT Intermittent gas lift is a cyclic process which can be divided into four stages. This paper describes a mechanistic model for the intermittent gas lift system based on fundamental physical principles. The model accounts for reservoir inflow performance, fluid properties, two-phase flow in the tubing, gas flow in the tubing-casing annulus, the performance of a gas-lift valve and a surface gas injection choke, and various intermittent gas-lift control mechanisms. The mathematical model consists of a system of ordinary and partial differential equations for each of the four stages. The resulting system of equations is numerically solved. The model is capable of generating the detailed cyclic characteristics and the overall performance of an intermittent gas-lift system. The model is validated using experimental data collected by a number of previous investigators. These data cover a wide range of operating conditions for intermittent gas lift The model's predictions compare well with the experimental measurements. The effects of various system parameters on the performance of the intermittent gas-lift system are studied using the model, and the most important controlling parameters are identified. The ability to identify these controlling parameters and their optimum values for a given system allows one to operate an intermittent gas-lift system more efficiently. Finally, the performance of intermittent and continuous gas lift are compared, and qualitative guidelines are outlined for selecting between intermittent and continuous gas-lift methods.